blk-cgroup: Remove unnecessary rcu_read_lock/unlock()

spin_lock_irq/spin_unlock_irq contains preempt_disable/enable().
Which can serve as RCU read-side critical region, so remove
rcu_read_lock/unlock().

Signed-off-by: Fanjun Kong <bh1scw@gmail.com>
Reviewed-by: Muchun Song <songmuchun@bytedance.com>
Acked-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/20220516173930.159535-1-bh1scw@gmail.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>